Films of thermoplastic polymer compositions have found many commercial uses, primarily in packaging. One large field of use of such films is in the production of medical and food packages which require certain film properties to avoid any indesirable leaching of plastic into food or the body.
A plastic film suitable for use in the fabrication of medical and food packages must possess a variety of good physical and mechanical properties. The film should be strongly resistant to tearing and exhibit good tensile strength. Clarity or transparency is another desired property. It is advantageous if heat sealing of layers of the film may be conducted over a broad range of sealing temperatures while obtaining good seal strength. To be favorable economically, the film should be readily processable at high product speeds (line speeds) and an increase in film gauge should not be required to maintain satisfactory physical and mechanical properties. Most important, the film should contain low ether solubles, and thus low extractables, to prevent undesired contamination of the saline or food, for example.
Films for use in packaging and sacking are conventionally produced by the well known blown-bubble extrusion process. The process is briefly described, for example, in "Encyclopedia of Chemical Technology"; Kirk-Othmer, 2nd Ed., 1966, Vol. 9, pp 235-236. The production of blown film from commercial film grade polybutylene resins is described in Technical Bulletin SC:391-79 of Shell Chemical Company, entitled "Processing Shell Polybutylene Film Grade Resins", published May, 1979. Conventional nomenclature frequently refers to films of greater than 10 ml thickness as sheet, although the process for producing them is called blown film extrusion, even for production of such sheets of up to 40 mil thickness.
In current commercial practice, low density polyethylene (LDPE) in the form of biaxially oriented films is the most widely used material for the production of packaging containers. However, LDPE film has relatively low tear strength and too high an ether solubles content. Isotatic poly-1-butene and its copolymers with small amounts of other monomers, commonly referred to as polybutylene homopolymers and copolymers, are superior to LDPE in respect to tear strength and ether solubles. This makes is possible to produce article of a given strength suitable for medical and food packaging from films of significantly lower gauge when using polybutylene rather than LDPE.
Copolymers of butene-1 with small amounts of ethylene, hereafter referred to as conventional butene-1-ethylene copolymers or conventional PBE, are products of commence which have been employed for production of film by the blown-bubble extrusion process. However, heat seals between layers of unmodified and uncompounded conventional butene-1 polymers such as PBE have an unsatisfactory seal strength. Incorporating 1 to 5 percent of LDPE into conventional polybutylene, as disclosed in U.S. Pat. No. 3,634,551, improves heat seal strength somewhat. However, blends of butene-1 polymers and LDPE form films which can only be heat sealed over a relatively narrow range of temperatures and do not possess adequately low levels of extractables.
It has been found that the balance of properties of films made from conventional butene-1-ethylene copolymer can be improved by making them from a blend of PBE with about 2 to 9% by weight of polypropylene (PP). Such films have improved heat sealing properties, as well as quite low ether solubles content. A further improvement, resulting in improved processing properties, is provided by incorporating a small amount, about 0.05 to 1.5% by weight, of high density polyethylene (HDPE) in the PBE-PP composition. These improvements are disclosed and claimed in U.S. Pat. Nos. 4,316,970 and 4,354,004, respectively. A novel, stereoregular, elastomeric polybutylene is described in co-assigned co-pending U.S. Ser. No. 369,388, filed Apr. 19, 1982.